Thermal treatment of some breast cancers may become an effective, minimally invasive, therapeutic tool given proper guidance of the thermal deposition. Proper guidance implies that the diseased tissue must be clearly identified and heated to an appropriate temperature for an appropriate length of time to insure effective cell destruction while cell damage to neighboring, healthy tissue is minimized to as great a degree as possible. Clearly an advantage can be gained if the thermal treatment can be combined with an imaging modality capable of monitoring the tissue temperature changes which occur during the heating process in order to maximize damage to diseased tissue while minimizing damage to healthy tissue. We believe that Magnetic Resonance Imaging (MRI) provides such a modality but that the full potential of MR for guidance of thermal therapy remains largely untapped due to the need for further technical development. For instance, a major limitation with current state-of-the art MR thermometry pulse sequence design is the dependence on baseline subtraction methods for temperature change measurement. Interscan motion, largely unavoidable in clinical situations, makes the dependence on baseline subtraction methods highly problematic for robust temperature monitoring. Another limitation, particularly pertinent to the breast, is interference from the lipid signal which confounds MR temperature measurements made on the assumption that the signal arises solely from the water resonance, the current method of choice in most experimental MR thermometry studies to date. The focus of this proposal is to develop novel MR pulse sequences which take advantage of state-of-the-art gradient hardware to overcome these limitations so that MR guidance of breast tumor ablation can become an efficient therapeutic reality. A cornerstone of the work is that the temperature sensitive water frequency can be referenced to the temperature insensitive fat frequency to allow for internally referenced temperature change measurements. It is proposed that this novel approach will simultaneously solve problems associated with interscan motion and baseline subtraction methods as well as lipid contamination. The proposal focuses on the development and testing of techniques for MR guided thermal therapy in the breast. These techniques have some unique properties which may, however, prove useful in other areas of medical imaging such as breathhold spectroscopic studies in the abdomen or functional MRI studies of the brain.